1) Field of the Invention
The present invention relates to a cooling system and, more particularly, to a cooling system employing a pulsating heat pipe for cooling a printed circuit board.
2) Description of Related Art
In avionics and other applications, printed circuit boards (“PCB's”) are commonly mounted within a metallic chassis box. The heat generated by electronic devices carried by the PCB is dissipated by transfer to a metal (aluminum) wall of the chassis box through the PCB's. The heat is then sent to an external heat sink by conduction through the metal chassis wall, and is finally taken away by either cool air circulating about the heat sink or a cold plate. Because of high thermal resistance in the heat transfer path, the waste heat load generally increases with further operation, which in turn, leads to a larger temperature gradient (ΔT) between the electronic devices that are generating heat and the heat sink. This larger ΔT may adversely affect the performance of the electronic devices. Consequently, a more effective heat transfer approach had to be developed to address these thermal problems.
One advanced heat transfer approach is pulsating heat pipe (PHP) technology. A PHP is made with a looped or unlooped meandering capillary tube that forms a closed circuit, as shown in FIG. 1. After partially filling a capillary tube that is maintained at a reduced pressure with liquid, the PHP reaches equilibrium by forming a plurality of vapor slugs separated by vapor bubbles, i.e., regions of saturated liquid. In operation, heat is introduced in an evaporating region and is withdrawn from a condensing region. At steady state, heat transfer is achieved by continuous oscillatory movements from the evaporating region to the condensing region, which is caused by instant pressure imbalance among different turns. Both phase change and sensible heat exchanges are considered to participate in the heat transfer.
Since the late 1990's, PHP operating characteristics and mechanisms have been studied extensively, and this technology has been applied to more and more fields. For instance, the PHP technology has been employed to provide avionics device cooling, which has provided a significant reduction in thermal resistance, and employs the systems disclosed in U.S. Pat. Nos. 4,921,041 5,697,428 to Akachi. In particular, and as shown in FIG. 2 of the present application, an avionics chassis box was disclosed to house printed circuit boards and to include a tunnel plate heat pipe within a wall of the chassis box. Another wall of the chassis box may be a cold plate connected to a condenser to output the heat that has been transferred to the heat pipe from the PCB's. By positioning the heat pipe within the wall, the thermal resistance is reduced and the heat transportation capability is increased.
With the introduction of new and additional electronic devices that generate more heat in avionics and other systems, more waste heat is generated by the printed circuit boards. For instance, a new generation of electronic systems utilized in an avionics chassis may need to dissipate up to 1000 watts of total waste heat during operation. For example, each power supply module may generate and need to dissipate approximately 100 watts. The increasing heat dissipation requirements therefore cause a very large temperature gradient when traditional cooling solutions are utilized. Therefore, despite the above-mentioned improvements in avionics cooling including incorporating the PHP into the chassis box, additional improvements are desired that are capable of handling increasing amounts of waste heat.
It would therefore be advantageous to provide an improved cooling system for rapidly dissipating the increased amounts of waste heat generated by modern electronic systems. In this regard, it would be advantageous to provide a cooling system that provides a higher heat transfer capability and lower thermal resistance than conventional cooling systems. It would also be advantageous to provide a cooling system that offers a relatively simple construction and lower manufacturing cost than conventional cooling systems.